Planar and essentially rectangular fuel cell and fuel cell block

ABSTRACT

A planar, rectangular and water-cooled fuel cell includes a cooling element with a cooling chamber through which cooling water flows during the operation of the fuel cell. Cooling water does not flow through the cooling chamber in a homogeneous manner, normally resulting in local heating of the fuel cell in regions through which the cooling water flows through less frequently. A fuel cell is provided with a cooling element which includes an essentially rectangular cooling chamber with four corner regions, whereby the opening of the coolant flow is arranged in the first corner, the opening of a first coolant flow is arranged in a second corner and a second coolant flow is disposed in a third corner. The first coolant flow has a cross section Q 1  on the narrowest point thereof and the second coolant flow has a cross section Q 2  on the narrowest point thereof, the ratio of Q 1 /Q 2  being 7-25.

[0001] The invention relates to a planar and essentially rectangular fuel cell having a cooling element which has an essentially rectangular coolant space with four corner regions, the mouth of the coolant inflow being arranged in a first corner region, and the mouth of a first coolant outflow being arranged in a second corner region. In addition, the invention relates to a fuel cell block having such a fuel cell.

[0002] In a fuel cell, electrical energy and heat are generated by the combination of hydrogen (H₂) and oxygen (O₂) in an electrochemical reaction, the hydrogen and the oxygen being combined to form water (H₂O). A single fuel cell supplies an operating voltage of a maximum of 1.1 V. For this reason, a plurality of planar fuel cells are stacked one on top of the other and are combined to form a fuel cell block. By virtue of the fuel cells of the fuel cell block being connected in series it is possible for the operating voltage of the fuel cell block to be several hundred volts. A fuel cell in a fuel cell block comprises a diaphragm electrode unit which is also referred to as an electrolyte, electrode unit, and the composite printed circuit board which is adjacent thereto on both sides. The composite printed circuit board can be configured as cooling elements.

[0003] The technical implementation of the principle of the fuel cell has lead to different solutions, specifically with different types of electrolytes and operating temperatures between 80° C. and 1000° C. Depending on its operating temperature, the fuel cells are classified as low-temperature fuel cells, medium-temperature fuel cells and high-temperature fuel cells which are distinguished in turn by various technical embodiments. The heat which is produced in a fuel cell by the electrochemical reaction must be carried away from the fuel cell so that the fuel cell is not destroyed by overheating. In the case of a low-temperature fuel cell, this heat is usually carried away using a coolant circuit, the coolant, generally water, flowing through the fuel cell, absorbing heat there and giving off the heat outside the fuel cell. For this purpose, the fuel cell comprises a cooling element which can be used either for cooling the fuel cell or else for heating the fuel cell, for example when the fuel cell block is started up. The coolant element has a coolant space through which the coolant, generally the cooling water, flows while the fuel cell is operating. The coolant space has a coolant inflow and a coolant outflow, the coolant inflow and the coolant outflow being arranged in such a way that the stream of coolant which flows from the inflow to the outflow cools the fuel cell as uniformly as possible.

[0004] EP 0 591 800 B1 discloses a cooling element which is composed of two plates and has a rectangular coolant space, the inflow and the outflow for the coolant being arranged in corner regions of the coolant space which are diagonally opposite one another. When cooling water flows through such a coolant space, the centre region of the coolant space is effectively cooled, but only a small amount of cooling water flows through the corner regions of the coolant space which are not adjacent to the inflow or outflow. This results in the fuel cell being heated to a greater degree in these corner regions than in its central region which adjoins the central region of the coolant space. In an extreme case, such defective conveying away of heat from the corner regions through which there is a weak flow leads to the electrolyte diaphragm of the fuel cell being destroyed at these points.

[0005] The object of the present invention is therefore to disclose a fuel cell in which heat is conveyed away from the elements of the fuel cell which are adjacent to the coolant space in a homogenized way in comparison with the prior art. In addition, the object of the present invention is to disclose a fuel cell block with a fuel cell with such improved conveying away of heat.

[0006] The first mentioned object is achieved by means of a fuel cell of the type mentioned in the beginning which, according to the invention, has a second coolant outflow in a third corner region of the coolant space, the first coolant outflow having a flow cross section Q₁ at its narrowest point, and the second coolant outflow having a flow cross section Q₂ at its narrowest point, and the ratio Q₁/Q₂ being 7 to 25.

[0007] By means of a second coolant outflow in a further corner region, an improved flow through this corner region which otherwise has a weak throughflow is achieved. As a result, the time for which the coolant water is present in this corner region is reduced, enabling it to absorb more heat there from the components of the fuel cell which are giving off heat, and the conveying away of heat by the cooling water from the fuel cell is thus homogenized. The cooling element can be configured with only a single second cooling outflow in a corner region or else with two second coolant outflows in two different corner regions. The second coolant outflow (or the second coolant outflows) is configured in such a way that considerably less coolant can flow out of the coolant space of the coolant element through said outflow than from the first coolant outflow. As a result, the main flow of coolant through the coolant space from the coolant inflow to the first coolant outflow is not significantly disrupted.

[0008] Only a small part of the coolant is branched off from this main flow and directed through the second coolant outflow. This smaller amount is selected in such a way that it is sufficient to keep the corner region, through which there is otherwise a weak flow, at approximately the same temperature as the central region of the fluid space. It has been shown in trials that, with an essentially rectangular coolant space, a coolant flow of approximately 3 to 10%, in particular 4 to 7%, through the third corner region, that is to say through the second coolant outflow, is sufficient to cause heat to be conveyed away uniformly in this corner region in comparison with the central region of the coolant space. Depending on the configuration of the coolant outflows, such a flow to the second coolant outflow is achieved if the flow cross section Q₂ of the first coolant outflow is approximately 7 to 25 times as large as the flow cross section Q₂ of the second coolant outflow at its narrowest point. If the first coolant outflow is configured in the form of, for example, 20 individual small ducts, the second coolant outflow is expediently embodied in the form of, for example, only one such duct. If the first coolant outflow is formed, for example, from only a single duct, its flow cross section Q₁ at its narrowest point is expediently 7 to 10 times the flow cross section Q₂ of the second coolant outflow which is embodied as a single duct.

[0009] In an advantageous configuration of the invention, the first and second corner regions are arranged essentially diagonally opposite one another. The first corner region with the mouth of the coolant inflow, and the second corner region with the mouth of the first and large coolant outflow form the starting point and end point of the main flow of coolant through the coolant space of the cooling element of the fuel cell. If these two corner regions lie essentially diagonally opposite one another, the largest possible quantity of heat is transferred from the fuel cell into the cooling water by this main flow. The regions of the coolant space through which this main flow flows to the smallest degree are located in the two other corner regions of the coolant space which are opposite one another. However, in one of these corner regions, or both of these corner regions, a second coolant outflow is arranged through which the flow of coolant through the coolant space is homogenized to a high degree.

[0010] In a fuel cell which is operated geodetically in an essentially vertically arranged fashion in a fuel cell block, that is to say in such a way that the plane of the cells is oriented essentially perpendicular to the surface of the earth, air bubbles collect in the coolant space in the course of the operation, at the upper edge of the coolant space. In the case of a fuel cell which is provided for such operation, the third corner region with the second coolant outflow is expediently arranged at the upper edge of the coolant space. In such an arrangement, the air bubbles can emerge from the coolant space through the second coolant outflow, effectively avoiding overheating of the fuel cell at the upper edge of the coolant space.

[0011] The object which the fuel cell block is intended to solve is achieved by means of a fuel cell block with a fuel cell according to the invention as described above, in which the first coolant outflow opens into a first axial duct of the fuel cell block, and the second coolant outflow opens into a second axial duct of the fuel cell block, and the two axial ducts are connected to one another using a pressure equalizing line.

[0012] An axial duct is understood to be a duct which runs in the stacking direction of the fuel cells within the fuel cell block which is composed of a plurality of stacked fuel cells. It is therefore oriented in the axial direction of the fuel cell block. The cooling fluid is taken out of the fuel cell block through such an axial duct of the fuel cell block. The coolant circuit in a fuel cell system which comprises a fuel cell block is generally an open circuit in which the pressure of the fluid within the axial duct which carries away cooling water is dependent on the geodetic height at which the axial duct, or a line adjoining it, opens to atmospheric pressure. The pressure ratio between the fluid pressure in the first axial duct with respect to the fluid pressure in the second axial duct is thus dependent on where the two axial ducts, or a line which is connected to them, open into the open air. As the flow of cooling fluid through a coolant outflow is dependent on the pressure within the axial duct to which the cooling fluid opens, it is desirable for the fluid pressure within the first axial duct to be in a fixed ratio with respect to the fluid pressure within the second axial duct. This is because it is only this way that it is possible to ensure that the flow of fluid through the first coolant outflow is in a predeterminable ratio with respect to the flow of coolant through the second coolant outflow. This ratio would thus be independent of the conveying of coolant from the fuel cell block into the fuel cell system. As a result of a pressure equalization line between the two axial ducts, the pressure in the two axial ducts is essentially always the same. As a result, the flow ratio through the two fluid outflows is always strictly defined and independent of the opening of the axial ducts into the open air. Uniform conveying of heat out of the fuel cell into the coolant, and thus a uniform temperature within the fuel cell are thus achieved.

[0013] The pressure equalizing line can be embodied in the form of a line, but it can also be equally well formed by a duct in the fuel cell block which connects the two axial ducts to one another. Such a duct may be arranged, for example, within the end plate or connecting plate of the fuel cell block or within an intermediate plate between the fuel cell block and a humidifier which is adjacent to it.

[0014] Exemplary embodiments of the invention are explained in more detail with reference to two figures, in which:

[0015]FIG. 1 shows a section through a cooling element of a planar and rectangular fuel cell;

[0016]FIG. 2 is an exploded diagram of a fuel cell block in a schematic view.

[0017]FIG. 1 represents a section through a cooling element 1 of a planar and rectangular fuel cell, the fuel cell comprising, in addition to the cooling element 1, a diaphragm electrode unit (not shown in FIG. 1) which is arranged underneath the cooling element 1 in terms of the view in FIG. 1. The cooling element 1 comprises a coolant space 3 and four corner regions 5 a, 5 b, 5 c, 5 d. The mouth of a coolant inflow 7 a is arranged in a first corner region 5 a, the coolant inflow 7 a connecting the coolant space 3 to the axial duct 9 a. A first coolant outflow 7 b, which connects the coolant space 3 to a first axial duct 9 b, opens into a second corner region 5 b. In a third corner region 5 c of the coolant space 3 there is the mouth of a second coolant outflow 7 c which connects the coolant space 3 to a second axial duct 9 c. The coolant space 3 has a fourth corner region 5 d to which, however, neither a coolant inflow nor a coolant outflow opens.

[0018] The fuel cell, and with it the cooling element 1, are planar in the plane of the paper of FIG. 1. The axial ducts 9 a, 9 b and 9 c run perpendicularly to the plane of the fuel cell, that is to say perpendicularly to the plane of the paper. During the operation of the fuel cell, cooling fluid, for example water, flows out of a supply device assigned to the fuel cell and through the axial duct 9 a to the cooling element 1 of the fuel cell. It flows through the coolant inflow 7 a and passes into the first corner region 5 a. The greater part of the cooling water flows through the coolant space 3, flows through the second corner region 5 b and then the first coolant outflow 7 b, and passes there to the first axial duct 9 b through which it is directed away from the fuel cell. A small part of the cooling water which passes through the coolant inflow 7 a into the coolant space 3 flows along the upper edge 11 of the coolant space 3 and passes into the third corner region 5 c. From there it flows through the second coolant outflow 7 c, passes into the second axial duct 9 c and is also directed away from the fuel cell through said axial duct 9 c. Air bubbles which collect in the coolant space 3 of the cooling element 1 are driven through the effect of the gravitation to the upper edge 11 of the coolant space 3. This air is driven largely through the second coolant outflow 2 out of the coolant space 3 by the cooling water, and into the axial duct 9 c from where it is expelled from the fuel cell.

[0019] The arrangement of the second coolant outflow 7 c in the corner region 5 c ensures that the warm water which collects along the upper edge 11 is conveyed away. As a result, reaction heat which is generated in the fuel cell is given off uniformly to the cooling water within the cooling element 1 in a regional fashion. Regional overheating of the fuel cell is thus effectively avoided.

[0020] The fuel cell, and with it the cooling element 1 are configured to be operated arranged in a fuel cell block in such a way that the upper edge 11 of the coolant space 3 is arranged at the top in terms of gravity. As a result of this it is possible to dispense with a further coolant outflow or inflow in the corner region 5 d. Cooling water which is heated in the lower half of the coolant space 3 is driven upwards by convection and thus out of the corner region 5 d, as a result of which there is a continuous flow of cool cooling water through the corner region 5 d. A third coolant outflow or a second coolant inflow in the corner region 5 d is thus not necessarily required.

[0021] The coolant outflows 7 b and 7 c are each configured as a single duct with a rectangular cross section. The flow cross section Q₁ of the first coolant outflow 7 b has seven times the cross sectional area in comparison with the flow cross section Q₂ of the second coolant outflow 7 c. Due to the geometry of the coolant outflow 7 b and 7 c, approximately 7% of the cooling water which enters the coolant space 3 through the coolant inflow 7 a flows through the second coolant outflow 7 c.

[0022] In FIG. 2, three fuel cells 21 of a fuel cell block 22 are illustrated in the form of an exploded diagram.

[0023] Each of these fuel cells 21 has a cooling element 23 and a diaphragm electrolyte unit 25. The cooling element 23 comprises a frame 23 b which is joined on each of its two sides by a plate 23 a and 23 c, respectively. The frame 23 b thus forms, with the two plates 23 a, 23 c, a cavity, the coolant space.

[0024] Each of the fuel cells 21 has in each of the corners a triangular recess. In fuel cells which are positioned one against the other, these recesses form axial ducts 27 a, 27 b, 27 c and 27 d which run vertically with respect to the plane of the cell. While the fuel cell block 22 is operating, cooling water flows from a supply device (not shown in more detail in FIG. 2) for the fuel cell block 22 into the inlet E of the axial duct 27 a of the fuel cell block 22. The cooling water is directed through the axial duct 27 a to the cooling elements 23 of the fuel cell block 22. In each case some of the cooling water flows through the coolant inflow 29 a of each cooling element 23 into the cooling space of the cooling element 23. The greater part of the cooling water flows through the coolant space in a diagonal direction and reaches the first coolant outflow 29 b, through which it flows and reaches the first axial duct 27 b. This cooling water from the cooling elements 23 of the fuel cells 21 of the fuel cell block 22 collects in the first axial duct 27 b and is carried out via the pressure equalization line 31, which is embodied as a line outside the fuel cell block 22, into the second axial duct 27 c in which it flows through the fuel cell block 22 and leaves it through the outlet A of the second axial duct 27 c. A small part of the cooling water flows from the coolant inflow 29 a of each cooling element 23 to the second coolant outflow 29 c, through which it is directed to a second axial duct 27 c. There, it is combined with the cooling water originating from the first axial duct 27 b, and flows to the outlet A of the second axial ducts 27 c.

[0025] The first coolant outflow 29 b of each coolant space is formed from twenty small ducts, only a few of which are shown in FIG. 2. The ducts connect the coolant space to the axial duct 27 b. The second coolant outflow 29 c of each coolant space is formed by a single duct which connects the coolant space to the second axial duct 27 c. The geometry of the ducts is in each case the same so that the flow cross section Q₁, composed of twenty duct cross sections, of the first coolant outflow has twenty times the area of the flow cross section Q₂, composed of only one duct cross section, of the second coolant outflow 29 c. The pressure equalization line 31 ensures that the fluid pressure within the axial ducts 27 b and 27 c is essentially the same. The pressure ratios within the axial ducts 27 b and 27 c thus do not favor any of the flows from the coolant inflow 29 a to the coolant outflows 29 b and 29 c. The amounts of coolant which flow off through the coolant outflow 29 b and 29 c are thus determined decisively by the flow cross sections Q₁ and Q₂, so that approximately 5% of the cooling water flowing through the coolant space leaves the coolant space through the second coolant outflow 29 c. 

1. A planar and essentially rectangular fuel cell (21) having a cooling element (1, 23) which has an essentially rectangular coolant space (3) with four corner regions (5 a, 5 b, 5 c, 5 d), the mouth of a coolant inflow (7 a, 29 a) being arranged in a first corner region (5 a), and the mouth of a coolant outflow (7 b, 29 b) being arranged in a second corner region (5 b), characterized by a second coolant outflow (7 c, 29 c) in a third corner region (5 c) of the coolant space (3), the first coolant outflow (7 b, 29 b) having a flow cross section Q₁ at its narrowest point, and the second coolant outflow (7 c, 29 c) having a flow cross section Q₂ at its narrowest point, and the ratio Q₁/Q₂ being 7 to
 25. 2. The fuel cell (21) as claimed in claim 1, characterized in that the first corner (5 a) and the second corner region (5 b) are arranged essentially diagonally opposite one another.
 3. A fuel cell block (22) having a fuel cell (21) as claimed in claim 1 or 2, in which the first coolant outflow (7 b, 29 b) opens into a first axial duct (27 b) of the fuel cell block (22), and the second coolant outflow (7 c, 29 c) opens into a second axial duct (27 c) of the fuel cell block (22), and the two axial ducts (27 b, 27 c) are connected to one another using a pressure equalizing line (31). 